WASHINGTON (CNN) -- A bill introduced in the House and Senate on Wednesday would require sex offenders to register with authorities before their release from prison and require two-time convicts to wear a monitoring device for life.

The legislation, co-sponsored by Rep. Mark Foley, R-Florida, and Sen. Orrin Hatch, R-Utah, also would designate the use of the Internet in committing a sex offense against a child as a federal crime.

It would require freed sex offenders to wear electronic tracking devices during any supervised release period. Convicts with two convictions would wear electronic monitors for life.

"They prey on our children like animals and will continue to do so until we stop them," Foley said at a news conference.

Foley said there are nearly 550,000 registered sex offenders in the United States -- and roughly 150,000 sex offenders who are not registered.

"We don't know where they are," he said.

Foley noted that there are more than 34,000 registered sex offenders in his state of Florida alone.

The bill follows recent slayings of two Florida girls, Jessica Lunsford and Sarah Lunde. In both cases, authorities have charged registered sex offenders with murder. Foley said the bill was not a "knee-jerk" reaction to those slayings.

"We've been working on this for more than seven months," Foley said.

Hatch also spoke at the news conference. "We're sick and tired of seeing our children molested, raped and killed," he said. "We're going to do everything in our power to make sure [sex offenders are] stopped."

Also at the news conference were the families of child abduction victims and John Walsh -- host of the television program "America's Most Wanted." His son, Adam Walsh, was abducted and killed in Florida in 1981.

"This piece of legislation would tighten the cracks," Walsh said, his voice breaking. "If we can't put these low-lifes in jail where they belong, at least we need to know where they are."

The measure, if passed, would require states to set up a searchable database of sex offenders and establish a federal DNA database for convicts.

It also would increase address reporting requirements for freed convicts, forcing them to update their registration information with authorities two to three times a year, depending on the offense, and within three days of any address change.